The present invention relates to sensor arrays of two dimensions or more for measuring a physical property which stimulates the sensor array at any position thereof, especially two-dimensional arrays for measuring pressure distributions.
A conventional method of measuring a physical property, for instance a pressure pattern, on two dimensional surfaces is to arrange multiple sensors in a matrix. The matrix is scanned using multiplexer to connect the individual sensor elements with a driving clement (for example, a current or voltage source) and a receiving element (for example, a voltage or current amplifier). Often a processor is used to feed the driving element and select the sensor one by one and to read the result of the individual measurements. Such multiplexer systems are known from U.S. Pat. Nos. 4,856,993, 5,756,904, 5,659,395, 5,505,072, 5,905,209 which are cited merely as examples.
These conventional systems all share a number of common limitations:
To optimize the required sensing hardware the matrix must be defined. Therefore different hardware and number of address lines have to be used for applications which require different sizes of the matrix.
For large surfaces the multiplexer control lines become long and accordingly reduce the scan speed.
For large surfaces the number of elements to be scanned becomes large and further reduces the scan speed.
The sensors, of which very large numbers are required, need to be constructed from a simple element suitable for mass production. This need for low manufacturing costs can result in the use of technologies for making the sensors with inherent quality control problems, for instance, the sensor include a sensitive layer made by screen printed pressure sensitive ink as described in International Patent Application WO 97/25379. The printing process can be very variable in quality. This often limits the sensing quality or accuracy, of the physical property to be measured.
The sensing element, often a resistor whose value changes due to the physical property of interest, needs to be isolated from its neighboring elements. If interconnections between sensors exist then crosstalk will appear causing ghost images and false measurements. This need to isolate increase the complexity of manufacture particularly with large size matrices. For instance, individual mounting of the sensors may be necessary which is costly.
Due to the required isolation between sensor elements the active surface, where the sensor measures the wanted physical property is usually only a fraction of the total surface from which results are to be recorded. Therefore a signal generated by a stimulation which lands in between sample points, can be measured only partially or is completely lost.
Preferably, each element contains some kind of switch so that each sensor can be switched on and off. This is conventionally done with a diode or transistor or by a mechanical switch action at the sensing point. If this switching is not present, the electronic output of an individual sensor which is not selected may not be zero and the leakage of thousands of such sensors together can still produce a considerable and often unstable signal. However, providing semiconductor active switching devices within the area of the matrix can increase the size of each sensor element and prevent manufacture of a flexible matrix or make its manufacture more difficult and/or costly. Further, electromagnetic screening of the select transistors may be necessary which increases the manufacturing complexity.
Often the processor needs to manipulate the output from the sensor matrix in order to obtain accurate and useful results, e.g. a sensitivity of a test signal has to be changed depending upon the range of values of the sensed property in order to compensate for the non-linearity of the sensing material. This results in a processor being an essential part of the complete device which increases the cost in some applications, e.g. simple surveillance where no analysis of the results is required.
Because of these limitations, it has been conventionally necessary to optimize tradeoffs to obtain a best fit to the requirements of a specific application.
An aim of the invention claimed is to provide a sensor array and a method of operating the same, in particular, for pressure measurement, which is an improvement over known sensor arrays.
It is a further aim of the present invention to provide a sensor array and a method of operating the same, which allows extension or change of shape of the array with minimum changes in the scanning electronics.
It is still a further aim of the present invention to provide a sensor array and a method of operating the same, which avoids the use of select switches at the sensing points.
The present invention may provide a multi-dimensional sensor array for sensing a physical property, comprising; a first plurality of generally parallel electrically conductive lines providing a first set of driven electrodes; a second plurality of generally parallel electrically conductive lines providing a second set of sensing electrodes, the sensing electrodes being at an angle to the driven electrodes; a sensitive sheet material in contact with the driven and sensing electrodes, the sensitive sheet having an electrical property which varies depending upon the physical property to be measured; a shift register unit comprising a linear array of one-bit registers and a data input to the first one-register of the shift register unit for inputting a serial bit stream, an output of each one-bit register being connected to one of the driven electrodes.
Current sensing circuits may be connected to the sensing electrodes, each current sensing circuit being connected to one of the sensing electrodes. The electrical property of the sheet material may be its sheet resistance. Alternatively, the electrical property may be is the transverse resistance or capacitance of the sheet material. The physical property to be measured may be a pressure applied to the sensor array. The object exerting the pressure on the sensor array may be a foot of a mammal. The sensing electrodes and the driven electrodes may both be in contact with one major surface of the sheet material. Alternatively, the sheet material may be sandwiched between the sensing and driven electrode sets. A clock circuit may be provided for supplying a clock signal to the shift register unit. An input circuit may be provided for applying a selectable serial pattern of bits to the data input of the first one-bit register of the shift register unit. The output from the current sensing circuits may be a video signal. The current sensing circuits may be current mirror circuits.
The present invention includes a sensor system for sensing a physical property comprising at least two sensor arrays joined at a common edge, each sensor array comprising; a first plurality of generally parallel electrically conductive lines providing a first set of driven electrodes; a second plurality of generally parallel electrically conductive lines providing a second set of sensing electrodes, the sensing electrodes being at an angle to the driven electrodes; a sensitive sheet material in contact with the driven and sensing electrodes, the sensitive sheet having an electrical property which varies depending upon the physical property to be measured; and a shift register unit comprising a linear array of one-bit registers and a data input to the first one-bit register of the shift register unit for inputting a serial bit stream, each stage being connected to one of the driven electrodes. The sensor arrays may be polygonal, e g. rectangular or square.
The present invention may provide a scanning system which uses one or more continuous layers of active sensing material. The sensing material can be selected for optimal measurement accuracy, avoiding the constraint of production requirements, for example the selection of manufacturing steps such as screen printing or individual mounting of sensors. Preferably, the sensitive areas can cover the area to be monitored in a substantially complete manner. Preferably, the scanning device for the sensor array produces an electronic rather then a physical isolation between the different sensors to be measured. Furthermore, in order to increase the scanning speed, multiple sensors may be scanned simultaneously until an active area is detected. Once detected the scan can concentrate on this area. In accordance with embodiment of the present invention, the scanning pattern of the sensor array may be changed dynamically to match the application.
The invention uses two different schemes to create an electronic, rather then a physical, isolation. For example, for a two dimensional surface to be measured, the surface is segregated by lines of electrodes in one dimension, for instance the y axis, which are maintained at ground level by the use of a current mirror. In the other direction, the x axis, the isolation is created by driving each intersecting line of electrodes which is to be excluded from the measurement (i.e. those lines which are not selected for measurement), to a zero voltage level.
In accordance with embodiments of the present invention, the sensor elements find themselves surrounded in all directions by a zero voltage field. Thus, whether these sensors are isolated or continuously connected or have a high or low impedance is irrelevant, no current will flow between points of equal voltage. The sensors which is neighbor lines driven by a sink (zero voltage) level will not contribute to any measurements and produce no leakage or capacitance effects.